a. Field of Invention
The invention relates generally to intake and exhaust systems for engines, and, more particularly to the control of an intake and exhaust system for a dual mode HCCI engine, which provides superior intake temperature control and homogeneity for engine operation in SI and HCCI modes, as well as during transition between SI and HCCI modes and vice-versa.
b. Description of Related Art
Compared to conventional engines, homogeneous charge compression ignition (HCCI) engines potentially have high efficiency, very low emissions of oxides of nitrogen (NOx) and particulates, and relatively low cost. HCCI engines however generally operate over the same operating range, with regard to speed and torque, as conventional SI or diesel engines, for achieving the same vehicle performance. Because HCCI is limited by harsh combustion at higher torques, it is common for the engine to employ both SI and HCCI combustion mode technology. At medium torque, the engine can operate in HCCI mode to achieve high fuel efficiency and low NOx emissions. At higher torques however, the combustion mode of the engine is typically switched to SI mode. Hence, there exists a need to improve upon existing, or alternatively, develop new methods and mechanisms for optimal engine operation in HCCI and SI modes, as well as during HCCI and SI combustion mode switching.
For combustion in an HCCI engine with a limited compression ratio, the gas temperature when the piston is at top dead center (TDC) should be high enough (i.e. about 1000K) for autoignition. The high temperature may be realized by using higher compression ratio and/or higher charge temperature before compression. In general, the lower the torque, the higher the intake temperature should preferably be.
For typical operation, the air-fuel mixture in a dual combustion engine in HCCI mode is diluted by air or by exhaust gas recirculation (EGR) through the use of high intake pressure (i.e. unthrottled operation at medium torque) to suppress NOx formation. In contrast, the intake temperature of a dual combustion engine in SI mode should preferably be low enough (i.e. close to ambient temperature) to avoid knocking, the compression ratio should be limited (i.e. CR<˜11:1), and the air-fuel mixture should preferably be at, or close to, stoichiometric. Thus, when a dual combustion engine is switching from HCCI mode to SI mode, the inlet temperature should preferably decrease quickly and the intake pressure should preferably also decrease quickly to restrict the intake airflow to form a stoichiometric mixture at a medium torque.
Another concern with dual mode HCCI engines is in the heterogeneity of the temperature distribution in a cylinder, which is known to affect the overall HCCI combustion rate. In HCCI mode, autoignition first starts at the hot regions within a cylinder, followed by autoignition in the cold regions. Thus, the overall combustion rate in a cylinder can be decreased due to heterogeneity of the temperature distribution within a cylinder.
In addition to the heterogeneity of the temperature distribution in a cylinder, effective intake temperature control is also of importance in a dual combustion HCCI engine. For example, in order to promote spontaneous HCCI combustion, it is necessary to maintain the homogeneous air/fuel mixture at a certain temperature so the active radicals will auto-ignite at the correct time in the cycle (i.e. proper combustion phasing). If the temperature is too high, the radicals can autoignite too early, creating excessive peak pressures, poor efficiency and other issues (i.e. engine damage). If the temperature is too low, the radicals may not combust at all, creating a misfire condition. Moreover, the required temperature varies with engine condition (i.e. speed, load, EGR, A/F ratio etc.).
Accordingly, the intake/exhaust system for a dual-mode HCCI engine can be relatively complicated and can include several factors which can effect the performance thereof, as evidenced by the aforementioned discussion.
Various related-art intake/exhaust systems for HCCI engines are known and disclosed, for example, in U.S. Pat. No. 6,295,973 to Yang, assigned to the assignee of the present application, and U.S. Pat. No. 6,276,334 to Flynn, U.S. patent application Ser. No. 09/573,743, and SAE paper No. 2002-01-0105, the respective disclosures of which are incorporated herein in their entirety by reference.
U.S. Pat. No. 6,295,973 to Yang, for example, discloses an intake system for an HCCI engine, which proposes using the waste thermal energy in the coolant and exhaust gases to heat the intake air and control the intake air temperature by mixing the heated and un-heated air streams with different mass ratios of the two air streams. U.S. Pat. No. 6,276,334 to Flynn discloses that varying the opening and closing timing of intake valves can be used to advance or retard the combustion event as desired. One drawback with the invention of Flynn is that it requires the continuous varying of valve timing, which can be difficult to implement for the HCCI and SI combustion mode transition region.
In order to improve upon the intake/exhaust systems of dual combustion engines, related inventions have tested HCCI-SI dual-mode engines with negative valve overlap. In these tests however, the engine geometric compression ratio remains low (CR<˜12:1), and the effective compression ratio for both HCCI and SI combustion remains essentially unchanged. Further, the results from these tests are not directly applicable to engines with a high geometric compression ratio (CR>˜15:1). Additionally, currently available HCCI-SI dual-mode engines rely fully on the hot residuals for autoignition, hence include a large negative valve overlap, which should preferably be avoided or minimized.
Lastly, in pending patent application Ser. No. 09/573,743, which is co-owned by the Assignee herein and the disclosure of which is incorporated by reference, HCCI-SI dual-mode engine operation strategies and mechanisms based on variable cam timing (VCT) are proposed. For the invention disclosed in application Ser. No. 09/573,743, the cam phasing speed can be of importance during combustion mode transition.
Based upon the aforementioned factors and concerns, there remains a need for a system for precisely controlling each cylinder's inlet air temperature and density in order to promote efficient and stable HCCI combustion, an intake system/strategy that can control both the overall air-fuel charge temperature and heterogeneity of temperature distribution in a cylinder, as well as a system which can facilitate HCCI-SI combustion mode transition, the system being structurally and economically feasible to manufacture and install, and the system efficiently and reliably achieving the aforementioned requirements for the relatively complicated operation of a dual-mode HCCI engine.